The 1st Battalion sailed on the Gascon on 14th December 1899, and arrived at the Cape on 7th January 1900. Along with the 2nd Bedfordshire, 2nd Worcestershire, and 2nd Wiltshire, they formed the 12th Brigade under Major General Clements. This brigade was intended to be part of the Vlth Division, but it was only for a short time under General Kelly-Kenny's command. The work of the brigade has been sketched under the Bedfordshire Regiment. In all that work the Royal Irish took a very prominent part, and frequently gained the praises of the commanders. In referring to the taking of Bethlehem on 7th July, Lord Roberts in his despatch of 10th October 1900 says, "On this occasion the 1st Battalion Royal Irish Regiment specially distinguished itself, capturing a gun of the 77th Battery RFA which had been lost at Stormberg". After being driven from Bethlehem the Boer army, about 7000 strong, retired into the Brandwater basin, where it was hoped they would be captured. On 9th July Clements' brigade had to go back towards Senekal for supplies. If this had not been necessary it is possible Steyn and De Wet would have found it impossible to have broken out of Slabbert's Nek, as they did on the night of the 15th with 1600 men and several guns.
On the 23rd Clements and Paget joined hands, and on this date the Boers were driven from their extremely strong positions at Slabbert's Nek. Here again the Royal Irish greatly distinguished themselves. To quote from Sir Archibald Hunter's despatch of 4th August 1900, para 26: "Major General Clements directed his troops to bivouac on the night of the 23rd on the positions they had gained, and at 4.30 am on the 24th Lieutenant Colonel Guinness with four companies Royal Irish and two companies 2nd Wiltshire, favoured by some clouds which obscured the crest, was able to gain a ridge to the west of and overlooking the enemy's position". Para 28: "Major General Clements reports that the position occupied by the Boers, who brought several guns and pom-poms into action, was one of great strength, and the fact that his turning movement was directed over ground 1500 to 2000 feet high is sufficient to explain the arduous nature of the operations". Sir Archibald having succeeded in closing the other exits, Prinsloo and over 4000 men surrendered on 30th July. Shortly after this the 12th Brigade was broken up. Clements was given command of a district in the Megaliesberg, but only one of his old regiments was left with him, the 2nd Worcestershire.
The fighting of the Royal Irish was not yet over. Along with a Scottish regiment which as yet had done nothing, the 1st Royal Scots, and one which had done an immense deal, the 1st Gordons, they were brigaded under Smith-Dorrien and placed under the divisional command of Ian Hamilton in order to advance on Lydenburg via Dulstroom, and so help Buller, who had been brought to a standstill at Badfontein. In this operation the Royal Irish again did well, their work being favourably mentioned in Lord Roberts' despatch of 10th October 1900.
The battalion was taken to Pretoria to represent Ireland at the ceremony of proclaiming the annexation of the Transvaal on 25th October 1900.
Ten officers and 17 non-commissioned officers and men were mentioned in Lord Roberts' final despatch.
When Belfast was attacked on the night of the 7th-8th January 1901 the Royal Irish provided part of the garrison. After severe fighting, the attack, which had been favoured by a dense mist, was driven off. The battalion lost 9 men killed and over 20 wounded. The praises of an enemy may be discounted by some, but it is at least worth noting that General Ben Viljoen in his book, when dealing with these attacks, mentions that the Royal Irish Regiment were the defenders, and says, "of which regiment all Britain should be proud". He also praised the 1st Gordons.
On the night of 7th January 1901 Private J Barry of the Royal Irish Regiment won the Victoria Cross under the following circumstances. The picquet to which he belonged was rushed by an overwhelming force of the enemy during a dense fog. The officer was killed and most of the men were killed, wounded, or taken prisoners. Private Barry succeeded in disabling the maxim by discharging his rifle into the mechanism, although he had been threatened by the enemy with instant death if he interfered with the gun. He carried out his object, but fell riddled with bullets. The Cross was handed to his wife. A writer in 'The United Service Magazine' for July 1903 said, "This is perhaps the finest exhibition of conspicuous bravery and devotion to country the war has produced".
In 1901 the battalion supplied the infantry of Colonel Park's column, one of those which operated in the Eastern Transvaal with much success under Sir Bindon Blood. After that, and until the close of the war, they were on garrison duty at Lydenburg, and also took part in many expeditions under General F W Kitchener and other commanders. A party from the battalion under Major Orr gained great praise for their capture of General Viljoen on 25th January 1902.
The Mounted Infantry of the Royal Irish did much excellent work, and were present in the very successful action at Bothaville, 6th November 1900 (see Oxford Light Infantry).
In the final despatch 7 officers and 6 non-commissioned officers and men of the battalion were mentioned.